Automatically actuated clutches can find application in vehicles equipped with different transmissions. Differentiation of the transmission thereby relates not only to the degree of automation, but also to the design, so that the transmission, for instance, can involve an automated manual shift transmission, an uninterruptible manual shift transmission, a parallel shift transmission or a transmission with continuously variable change of the transmission ratio. In such transmissions, the clutch is generally executed in the form of a friction clutch.
Both when starting a vehicle and when restoring the power connection between the engine and the drive train, after a gear shift process, clutch torque is transmitted via the clutch in the event of a difference in speed between the input- and the output side of the friction clutch. In the course of this, frictional power is introduced as a product of clutch torque and the differential angular velocity between the clutch's friction surfaces, which leads to an increase in the temperature of the friction surfaces and thus of the entire clutch.
The abrasion of the clutch's friction surfaces depends on the magnitude of input energy and the temperature. Besides appropriate abrasion of the friction surfaces, secondary effects can also occur in the form of mechanical deformation of the friction surfaces and changes of the coefficient of friction of the friction surfaces, so that, for instance, in the event of a decrease of the coefficient of friction of the friction pair, an increase of the slip phase can occur and hence lead to an increase of input energy.
Therefore, particularly with dry-running clutches, it is important not to allow the energy dissipated in the clutch to be high.
For vehicles with automatically actuated clutches, direct activation of the clutch has been omitted from the driver's influence, however the driver can influence the abrasion characteristic of the clutch through his/her driving style.
For instance, it is possible that long lasting creeping motion of the vehicle moving uphill or a delayed starting process, whether noticed or unnoticed by the driver, leads to an increase in energy dissipated in the clutch. In addition, holding a vehicle at uphill by using the gas pedal leads to an increase in energy dissipated in the clutch, wherein, for instance, also forgetting to release the handbrake when starting the vehicle also increases the energy input.
Besides these causes of an increased energy dissipated in the clutch, which result from the driver's action, an increase in energy dissipated in the clutch can also result from a malfunctioning system for actuating the automated friction clutch, for instance, owing to detuning in the hydraulic line for clutch actuation, for instance, due to leakage. All these are only an example of causes of increased loading in the form of increased energy input into the friction clutch.
From this situation, EP 1 616 770 B1 discloses a process for protecting an automatically actuated clutch against overload. In particular, it describes how the drive torque of the engine is reduced when a starting process takes place against the operating brake.
The possibility that the driver can activate the gas pedal—also called accelerator—and the brake pedal concurrently, for instance, when starting the vehicle against the brake action—also called stall—entails a significant source of danger for systems with automated clutches. For a vehicle that is halted by a handbrake, in the worst case, maximum engine torque at high-slip-rotation-speeds can be transmitted by the clutch, which may quickly lead to clutch damage. Such a misuse situation can last arbitrarily long. It is therefore required indeed that the XSG-software incorporates a suitable strategy for protecting the clutch.
The protection strategy currently implemented in vehicles essentially triggers limited intervention in the engine upon detecting a situation in which the torque developed by the engine is limited to a possibly small value, so that incurred slip-power remains uncritical. The limitation of engine torque will again be relieved as soon as the brake pedal is released.
The above-described strategy has disadvantages when starting to drive uphill. When the driver holds the vehicle with the brake then accelerates and releases the brake slowly, he does not expect the vehicle to roll backwards. The protection strategy, though, prevents the development of engine torque and because of clutch torque so long as the brake is fully released. A heavily loaded vehicle on a steep hill can suddenly roll backwards. Rolling backwards on a hill is unexpected for the driver and is unpleasant because he was initially just trying to prevent it by applying the brake.
The above-described conflict between clutch protection on the one hand and the desired dynamic starting characteristic on a hill has been solved with a compromise in today's software: the applicable engine torque limit at approx. 40 Nm is set so high such that given moderate values for vehicle weight and road inclination, back rolling no longer occurs, and on the other hand, the incurred friction power of approx. 6 kW, when the gas pedal for acceleration and the brake pedal for braking are pressed, is still acceptable.